1. Field of the Invention
This invention relates to means for preventing fluid leakage from a compressor casing's shaft opening and more particularly to a sealing mechanism which utilizes a static seal piston to trap the fluid in the compressor casing while the compressor's dynamic sealing mechanism is inoperative.
2. Description of the Prior Art
In axial compressors the rotor shaft ends must be brought out of a fluid-tight enclosure or casing, necessitating the use of some means to prevent escape of compressed fluid from the casing along the shaft to the atmosphere. When the compressed fluid is natural gas, it is necessary to prevent leakage from the compressor to the atmosphere because of the high cost of natural gas to prevent ingress of air into the casing because an air-natural gas mixture is highly combustible. Ring seals and rubbing seals have often been used to trap the compressed fluids in casings, but were judged not to be entirely satisfactory for all modes of operation of nautral gas compressors.
The ring seal utilizes a set of annular plates which surround the shaft and are supplied with oil under pressure. The oil is distributed around the shaft by the annular plates with some oil forced both ways along the shaft through a small clearance between the plates and shaft, thus preventing escape of compressed fluid from the casing. The ring seal containment method functions satisfactorily if the oil supplied to the ring seals is maintained at a higher pressure than the compressed fluid pressure in the casing. The ring seal was also commonly used during non-rotation of the shaft, but that required continuous seal oil pump operation when the compressor was not functioning.
The rubbing seal method for preventing leakage has been extensively used for both the static and dynamic sealing modes. The rubbing seal method utilizes a material such as graphite which is held in contact with the shaft or a collar attached thereto. The interface between the sealing material and collar requires lubrication to prevent excessive heat generation, material failure, and rapid depletion of the sealing material. Even with lubrication, the rubbing seal requires frequent and periodic replacement. The rubbing seal method has historically been used primarily in applications having relatively small shafts.
When compressor speed is reduced below the normal 3600 revolutions per minute in a closed loop refrigeration system utilizing natural gas as the refrigerant, the pressure within the compressor can increase from the standard 68 psig to 480 psig due to the change in phase from liquid to gas of a large portion of the natural gas in the refrigeration loop. Since the ring seal method requires the seal fluid pump to provide a higher discharge pressure than that of the natural gas, the seal fluid pump would consume a great deal of power when the compressor was not in the normal mode of operation. Another factor which makes the ring seal method unattractive for compressor shut-down is the enormous volume of sealing fluid required for a large, 155,000 horsepower compressor shaft. The large shaft size and accompanying surface interface speeds also eliminate the rubbing seal method from consideration because of the high wear rate and degree of leakage along the shaft.